1. Field of the Invention
The present invention relates to an automatic gain control apparatus and method. More particularly, the present invention relates to an automatic gain control apparatus and method for a compressed mode in a High-Speed Packet Access (HSPA) system supporting Multiple Input Multiple Output (MIMO).
2. Description of the Related Art
Mobile communication systems are evolving into high-speed and high-quality wireless packet data communication systems for data services and multimedia services. High-Speed Packet Access (HSPA) has been developed to support a high-speed and high-quality wireless packet data transmission service. The HSPA introduces a Multiple Input Multiple Output (MIMO) technology that uses a multiple transmission/reception antenna. The MIMO is a technology for simultaneously transmitting a plurality of data streams through a same resource, using the multiple transmission/reception antenna.
A basic Universal Mobile Telecommunications System (UMTS) supports a compressed mode for determining signal states of several neighbor cells consistently, in order to determine a target cell. In the UMTS system, a Transmission Gap (TG) denotes a Discontinuous Transmission (DTX) gap caused by the compressed mode. During the DTX gap, a Radio Frequency (RF) of a terminal shifts to a frequency band of a cell intended for determination, and performs diverse forms determinations required by an upper layer of the UMTS.
Accordingly, a receiver supporting MIMO supports antenna diversity. That is, the receiver determines power of an input signal from an antenna path and performs Automatic Gain Control (AGC).
FIG. 1 is a diagram illustrating a conventional operation process of an HSPA receiver supporting MIMO.
Referring to FIG. 1, if an inter-frequency determination signal is received, at time ‘t4’, the receiver stores a previously accumulated power level of a received signal in a buffer and completes a shift to a different frequency. At time ‘t5’ when a frequency synthesizer converges, an accumulator receives a new power level and initiates an accumulating operation.
The receiver then sets a large gain constant to increase a BandWidth (BW) of a loop. At time ‘t6’ after the lapse of a constant time, the receiver decreases the gain constant, decreases the BW and determines a power level of a received signal.
The receiver then completes inter-frequency determination and, at time ‘t9’, stores an accumulated value in a new buffer. At this time, during a return to an original frequency, the accumulator maintains a ‘hold’.
At time ‘t10’ when a frequency shift to the original frequency is completed, the receiver increases the gain constant again, increases the BW and at time ‘t11’, after the lapse of a constant time, decreases the gain constant again and decreases the BW.
FIG. 2 is a graph illustrating a conventional AGC simulation result of an HSPA receiver supporting MIMO.
In FIG. 2, it is appreciated that a power determination value operates with a large BW at time ‘t10’ and follows up to a current receive power. Up to time ‘t11’, the receiver has a stabilization time, and after time ‘t11’, a searcher and a rake initiate an operation.
At the time of a return to the original frequency after the inter-frequency determination, a receive signal power level of an accumulator of a ‘hold’ state is in an intactly accumulated state. Thus, returning to a signal intensity of a reference power level takes a long time.
Although the TG is generated due to the compressed mode, problems occur if data transmission/reception is discontinued or performance is deteriorated. Thus, in some cases, a transmission side may perform power compensation for a frame corresponding to the compressed mode.
In this case, there may be an increase or decrease of a receive power within a range of 2 dB to 3 dB (least) to 9 dB (worst). Further, in the case of Wideband Code Division Multiple Access (W-CDMA), the TG can have a period of a minimum of 3 slots. If the TG is over a frame, the TG can have a period of a maximum of 14 slots.
If the terminal moves at high speed during this period, a variation of power with a Base Station (BS) may increase. In order to make up for the increase, a method for quickly converging a receiving AGC (RX_AGC) and matching a reference power with a power level may be used, by increasing a gain constant and a BW. However, in this case, the converging into the reference power takes a long time.
Therefore, a need exists for an AGC apparatus and method in a MIMO system for reducing a search time in a compressed mode operation.